Rearrangements of the mixed lineage leukemia gene (MLL), the human homologue of the Drosophila gene Trithorax (trx), are associated with aggressive lymphoid and myeloid leukemias in both children and adults. Fusion of MLL to one of over 25 different translocation partners converts it into a leukemogenic oncoprotein. In addition internal tandem duplications of MLL occur in about 10% of acute myeloid leukemias with normal cytogenetics and are associated with a poor prognosis. A rapidly growing body of evidence suggests that MLL fusion proteins transform via increased expression of Hox genes, including Hox a9, which in cooperation with the Hox cofactor Meis1, promote leukemogenesis. Our studies of wild-type MLL indicate that it is a histone methyltransferase that dynamically regulates Hox gene expression. MLL binds directly to Hox promoters resulting in both histone acetylation and histone H3 lysine 4 methylation. MLL fusion proteins also upregulate expression of Hox genes and Meis1, which is likely the mechanism by which these proteins cause leukemia. These findings provide a strong rationale and experimental framework for defining how leukemogenic forms of MLL modify the histone code to deregulate Hox gene expression. In SA#1 we will define how wild-type MLL fusion modifies the "histone code" at target Hox genes, and determine how this code is differentially altered by MLL fusion proteins in hematopoietic cells. In SA#2 we examine how targeting of MLL to Hox promoters results in increased histone acetylation. Specifically we will determine if MLL affects recruitment of histone deacetylase containing corepressor complexes to target promoters and explore the role of a highly conserved domain of MLL in this process. In SA#3 we will examine whether coregulator recruitment changes during the downregulation of Hox a9 expression that occurs during myeloid differentiation and how this process may be perturbed by leukemogenic forms of MLL. These experiments will provide fundamental insights into mechanisms of transcriptional regulation by MLL and may facilitate the development of targeted therapies for acute leukemias with MLL rearrangements.